1. Field of the Invention
The present invention relates to a digital tone generator and especially to such a generator for use in digital telecommunication systems.
2. Description of the Prior Art
A basic form of a digital tone generator is illustrated and described in British Patent No. 1,325,193. Such a tone generator is employed instead of a conventional analog tone generator for various tone signals which are required in the operation of digital telecommunication systems. There is a need to generate various call processing tones, such as a busy tone, answer-back tone, etc., various dial tones and various test tones internally used by the telecommunication system for testing the availability and performance of redundant sub-units. Such tone generators may, therefore, furnish continuous tones or produce pulses of a discontinuous tone signal. Each tone may be composed of one or more frequency components according to national and international standards. Recent advances in the field of semiconductor technology offer possibilities to design digital tone generators requiring less space, being less costly and more flexible in use than their analog counter parts, so that they may be utilized to advantage in any application where analog tone generators have previously been employed.
The structure of a digital tone generator composed of components operating on a digital basis fits properly into a digital telecommunication system comprising a plurality of speech channels transmitting binary encoded information simultaneously in time and space multiplex. Furthermore, a digital tone generator can easily be adjusted to different applications to generate different sets of tones.
In addition, there are new areas of application for digital tone generators such as in connection with electronic music apparatus.
In many instances, it is desirable that a digitally generated tone does not begin or stop at its full loudness. When used in a telephone exchange system, "hard switched" tones would generate key clicks at the beginning and end of pulses of discontinuous tone signals. This should be avoided for the comfort of subscribers. Even more important, however, is a "soft switching" of pulses for obtaining a high reliability of digital telecommunication systems. Such systems include devices, such as filters, which react violently to sudden energy changes, tend to oscillate subsequently and need long recovery times until valid signal conditions are reached.
In digital tone generators which heretofore have been employed, the output level of the tone generator is adjustable so that a nominal loudness level can be achieved. However, these systems do not incorporate a separate control for individually adjusting the loudness level at the beginning and end of a tone pulse.
While the apparatus disclosed in the aforementioned British Pat. No. 1,325,193 is an attempt to reach a desirable solution to the problem, the apparatus described therein has some disadvantages which are desirable to avoid.